Optical discs represented by DVDs (Digital Versatile/Video Discs) are commonly used media capable of storing a large amount of digital data such as AV (Audio Video) data and computer data. For example, high-quality video which is over two hours in length is recorded on read-only optical discs and available on the market. In order to prevent such digital copyrighted works from being illicitly copied onto other recording media, a method known as content encryption is employed (See “Nikkei Electronics” Nov. 18 (1996): 13˜14).
In the above method, compressed digital content such as movie content is encrypted by the use of three-layered secret keys (title key, disc key, and master key) and recorded in a user information area accessible by a user. Of these secret keys, the master key, which is most important of all, is notified only to a licensed authorized manufacturer, whereas the disc key and the title key required for each DVD and title are encrypted on the basis of the master key and stored in a control information area inaccessible by a user. This prevents a user's access to the secret keys required for decryption, and therefore an unauthorized copying such as by means of file copying cannot be carried out.
However, the above method allows encrypted content to be decrypted and reproduced by ordinary reproduction apparatuses, when the whole contents of a recording area including the control information area in which the secret keys are recorded is illicitly copied onto another optical disc.
In response to this problem, there is a method for recording sub information onto an optical disc that stores main information such as digital content and the like by means of phase modulation to displace standard edge positions of recording marks that constitute the main information in the track direction by a minute amount (See Japanese Laid-Open Patent publication No. 2001-357533 and Japanese Laid-Open Patent publication No. 2002-203374).
FIG. 1 is a signal waveform diagram illustrating the above recording method.
In this method, PE modulation is performed on a sub information correlation random number sequence obtained by carrying out exclusive OR between a pseudo random number sequence (random number sequence) and sub information to be recorded. When a resulting PE modulated signal is “H”, the edge of a standard recording mark is advanced by a small amount (Δt) in the track direction, whereas when the PE modulated signal is “L”, the edge of a standard recording mark is delayed by a small amount (Δt) in the track direction, so as to record a modulated recording mark including main information and sub information onto an optical disc.
Accordingly, since sub information superimposed on the edges of modulated recording marks cannot be copied even if the whole contents of a recording area including the control information area is illicitly bit-copied onto another optical disc, it becomes impossible for an ordinary reproduction apparatus and the like to decrypt an illicitly copied optical disc, by recording information including a secret key as sub information. Furthermore, by setting the amount of mark edge displacement Δt to a value smaller than or equal to a value represented by the following equation, it is possible to minimize an influence on existing reproduction apparatuses which are not capable of reading out sub information:
                                                        (Allowable  jitter  of  optical  disc                        ⁢                                          )                            2                                -                                                                        (Jitter  only  of  main  information  on  optical  disc                        )                    2                    
FIG. 2 is a data structure diagram showing the data structure of an optical disc 2200 on which information has been recorded using the above method.
As FIG. 2 shows, the optical disc 2200 has a sector structure in which a certain number of data is included for recording main information. Moreover, respective ECC blocks j1, j2, . . . are made up of 16 sectors i0˜i15.
Each of the sectors i0˜i15 in an ECC block is comprised of 26 frames f0˜f25 made up of synchronization areas and data areas 2204 storing main information.
Each of the synchronization areas sy0˜sy25 is intended for generating a timing signal for reproducing their respective contiguous data areas 2204 on a byte-by-byte basis, and each data area 2204 is demodulated every 16-channel bits on the basis of the timing indicated by a timing signal so as to obtain main information. An address ID 2206 for identifying the sector i10 is placed at the top of a data area 2204 of the top frame f0, and such address ID 2206 is used at the time of reproduction to find a sector to be read out.
In FIG. 2, sub information sdb0˜sdb7 are superimposed only on recording marks in the data areas 2204 included in 24 frames f1˜f24 except for the top frame f0 (the frame that includes the address ID 2206) and the last frame f25 in the sector i10, and 1-bit sub information sdb0˜sdb7 are superimposed on a plurality of frames (three frames in FIG. 2). For example, sub information sdb3 is superimposed on recording marks that constitute data areas 2204 of the frames f10˜f12. As a result, 8-bit sub information is recorded per sector, and 16-byte sub information is recorded per ECC block.
As described above, the existing recording method utilizes the fact that main information is recorded in a distributed manner in a plurality of frames which are made up of synchronization areas and data areas, in order to correctly reproduce sub information subsequent to the synchronization areas even when bit misalignment occurs in the data areas due to flaw, dust, fingerprints and other factors on the surface of an optical disc, by detecting the synchronization areas and initializing the random number sequence always with an identical initial value.
Referring to FIG. 3, an explanation is given of the structure of an ECC block.
An ECC block is comprised of main information data 2301, a parity A2302, and a parity B2303. Assuming that consecutive 172 bytes are equivalent to one row in the main information data 2301 on an optical disc, 16-byte Reed-Solomon code parity A2302 is added to each 192-byte main information data made up of 172 columns, and an inner code 2304 is formed. Furthermore, 10-byte parity B2303 is added to each 172-byte data made up of 208 rows which is a combination of the main information data 2301 and the parity A, and an outer code 2305 is formed.
As shown above, an ECC block in an optical disc is made up of a product code of an inner code 2304 and an outer code 2305. Since the minimum distances of the outer code 2305 is 17 and the inner code 2304 is 11, the inner code 2304 is capable of correcting an 8-byte error and the outer code 2305 is capable of correcting a 5-byte error. Furthermore, by utilizing extinction correction, the inner code 2304 which is a vertical code, is capable of correcting a 16-byte error.
However, such existing technology, which uses a fixed value for initializing random number sequences as explained with reference to FIG. 2, has a problem that the whole sub information will be illicitly detected if those who illegally attempt to detect sub information discover an initial value that generates a random number sequence strongly correlated with a random number sequence which is actually used for the superimposition of sub information.
Furthermore, although main information can be reproduced even when a burst error occurs because of the reason that a burst error up to 16 rows can be corrected in the vertical direction through an error correction as explained in FIG. 3, sub information, which is recorded in consecutive frames, is likely to become unable to be reproduced due to bit misalignment or other factors. This means that there arises a problem that reliability concerning reproduction will be decreased.
In other words, the aforementioned existing example has a problem that sub information is vulnerable to unauthorized detection and is easy to become unable to be reproduced. This is the problem that decreases reliability.
The present invention has been conceived in view of the above problem, and it is an object of the present invention to provide an optical disc recording method and an optical disc reproduction method with increased reliability. More specifically, the first object of the present invention is to provide an optical disc recording method and an optical disc reproduction method capable of preventing unauthorized detection of sub information, and the second object of the present invention is to provide an optical disc recording method and an optical disc reproduction method capable of reproducing sub information even in the event of a burst error.